Beside control panels that are designed to work with OpenVZ, a lot of great tools have been released that allow you to centralized manage your servers in general. If you are in knowledge of a popular programming language like Ruby, Python or Perl it's easy to create your own OpenVZ modules for them.

Perl

(R)?ex [1] - With (R)?ex you can manage all your boxes from a central point through the complete process of configuration management and software deployment.

Python

Fabric [2] - Fabric is a Python (2.5 or higher) library and command-line tool for streamlining the use of SSH for application deployment or systems administration tasks.

Ruby

Capistrano [3] - Capistrano is a utility and framework for executing commands in parallel on multiple remote machines, via SSH.

If you have a working module for one of them or can suggest more such tools, feel free to add them!